1. Field of the Invention
The present invention relates to an arrangement for compensating the thermal characteristics of a sensor that detects displacement, torque, and pressure, using a change in the inductance of detecting coils.
2. Description of the Related Art
The following methods have thus far been available to inhibit a drift of magnetic sensor output voltage due to a temperature change:
1) Materials with nearly the same coefficient of thermal expansion are combined to fabricate the components, i.e., an object to be detected (core) and a detector (detecting coil), of a detecting section, which is one of mechanical factors causing an output voltage drift, so that a relative positional difference between the components may be reduced which is attributable to difference in thermal expansion between them.
2) To reduce the effect of temperature, a differential transformer is used which outputs according to a change in the inductance of the secondary coil due to core displacement, which change is detected as a change in voltage (induced electromotive force) induced by the primary coil.
The applicant has proposed in Japanese Patent Application No. 130363/1994 a steering torque sensor 100 shown in FIG. 10, used in an electrically powered steering system utilizing a change in coil inductance.
The steering torque sensor comprises as follows: An input shaft 101 and an output shaft 102, both of which are made of iron, are internally connected together, using a torsion bar 103, and a cylindrical core 107 is fitted over the input shaft 101 and the output shaft 102. Two detecting coils 105 and 106, supported by a housing 104, are disposed so that the detecting coils surround the core 107, the detecting coil 105 having the same number of wire turns as the detecting coil 106.
In the steering torque sensor 100, the torsion bar 103 undergoes elastic deformation, thus causing the input shaft 101 and the output shaft 102 to be displaced relative to each other, when steering operation provides the input shaft 101 with twisting torque.
The core 107, placed over the input shaft 101 and the output shaft 102, is provided with a longitudinal guide groove (not shown), which receives a pin on the side of the output shaft 102, and is arranged so as to be only movable axially. A slanted groove 108 is formed in a segment of the circumference of the core 107, into which groove a pin 109 of the input shaft 101 is fitted.
The above-described arrangement converts into the axial displacement of the core 107 that of the input shaft 101 and the output shaft 102 relative to each other due to twisting in the direction of rotation. The displacement of the core 107 causes a magnetic change around detecting coils 105 and 106, so that the inductance L.sub.1 of the detecting coil 105 and the inductance L.sub.2 of the detecting coil 106 change, both coils being vertically placed. The change in inductance is electrically extracted to detect steering torque.
In the above-described art, if measures such as selection of materials with nearly the same coefficient of thermal expansion for the components of a sensor are taken to arrange the sensor so that a relative positional difference may hardly occur, it is difficult to completely eliminate a mechanical positional difference while satisfactory sensor performance is ensured.
In the steering torque sensor 100 shown in FIG. 10, a temperature change causes a relative positional difference between the detecting coil 105, the detecting coil 106, and the core 107. As a result, the inductance L.sub.1 of the detecting coil 105 and the inductance L.sub.2 of the detecting coil 106 change.
The thermal characteristics of the detecting coils 105 and 106 themselves, magnetic changes (changes in permeability and eddy current loss) due to physical phenomena occurring around the detecting coils 105 and 106, and others also change the inductance L.sub.1 of the detecting coil 105 and the inductance L.sub.2 of the detecting coil 106.
It is therefore difficult to virtually nullify a drift, caused by such mechanical and electromagnetic properties, of voltage occurring as sensor output.
The present invention was made to correct the above-described disadvantages of the prior art.
It is an object of the present invention to provide an arrangement for compensating the thermal characteristics of a sensor, arrangement that can reduce an output voltage drift due to a temperature change to cover sensors with increased accuracy.
It is another object of the present invention to provide an arrangement for compensating the thermal characteristics of a sensor, arrangement that allows a stable, accurate steering torque sensor, committing few errors, for use in an electrically powered steering system to be obtained.